1. In chronic unrestrained cats the electrospinogram (ESG) was characterized by fast activity of 10-30 $\mu V$ amplitude during relaxed wakefulness and synchronous sleep. During desynchronized sleep groups of slow waves positive or diphasic at 3-5 c/s, of 50-200 $\mu V$ amplitude appeared constantly associated with the most intense bursts of REM. The ESG background fast activity also increased both in amplitude and frequency during the above mentioned slow potentials. These phenomena were not influenced by antero-posterior bilateral rhizothomy while disappeared after chronic section of the spinal cord at T₉. 2. The P wave of CDP, evoked in the cord corsum by a train of impulses at 300/sec to group I fibers of PBST nerve did not show any tonic change when the animal passed from wakefulness to synchronous and desynchronized sleep. A phasic reduction or abolition of P wave occurred in coincidence with the ESG slow waves synchronous with the most intense bursts of REM. A similar reduction could be seen during arousal. The phasic variations of CDP disappeared after chronic section of the spinal cord at T₉. 3. The antidromic spike evoked in GS nerve by microelectrode stimulation of its Ia afferent terminals into the spinal cord did not show any tonic change in amplitude during the sleep-wakefulness cycle. The conditioning PBST-GS curves were similar to those reported in acute animals. They were not influenced by the sleep-wakefulness rhythm. Phasic increase of GS response amplitude occurred synchronously with the slow waves of the ESG during paradoxical sleep. Similar phasic increase was found during arousal. 4. The phasic contemporary appearance during REM of these phenomena, viz.: (i) slow waves in the ESG: (ii) increased fast ESG background activity; (iii) P wave reduction or abolitipn; (iv) increase of GS antidromic spike amplitude, suggest that PAD of group Ia fibers induced by supraspinal structures may take place in the spinal cord during desynchronized sleep and be responsible for phasic presynaptic inhibition of monosynaptic reflex.